The present invention relates to a magnetic recording medium and a magnetic storage apparatus and, more particularly, to a magnetic recording medium having a recording density not less than 50 giga bits (Gb)/in2 and a magnetic storage apparatus in which the magnetic recording medium is incorporated.
Since 1998, the areal recording density of a magnetic storage apparatus is raised by an increase rate of about 100%/year. However, as the areal recording density is raised, a problem of so-called thermal fluctuation comes to be caused in which magnetically recorded data disappear due to surrounding heat, so that the achieving of an areal recording density more than 50 Gb/in2 is considered to be difficult insofar as a conventional longitudinal recording method is concerned.
On the other hand, a perpendicular recording method has, unlike the longitudinal recording method, the characteristic that, the more the linear recording density is raised, the more a demagnetizing field occurring among adjacent bits decreases, so that a recorded magnetization can be maintained to be stable. Further, in the perpendicular recording method, a high head magnetic field can be obtained by providing below a perpendicular recording layer a soft magnetic underlayer having a high permeability, whereby it becomes possible to enhance the coercivity of the perpendicular recording layer. Because of these reasons, the perpendicular recording method is considered to be one of effective means for overcoming the thermal fluctuation limit occurring in the longitudinal recording method.
In the perpendicular recording method, the combination of a single-pole-type head and double-layer perpendicular recording media each comprising a soft magnetic underlayer and a perpendicular recording layer is effective in realizing the high desity recording. However, since the double-layer perpendicular recording medium has the soft magnetic underlayer having a high saturation magnetic flux density (Bs), there occur such problems as a stray field occurring from the domain walls of the soft magnetic underlayer is observed as spike noises and-as the recorded magnetization disappears due to the domain wall motion. To overcome these problems, it is proposed to provide a hard magnetic pinning layer between the soft magnetic layer and a substrate so that the magnetization of the soft magnetic layer is directed in one direction, as proposed, for example, in JP-A-7-129946 and JP-A-11-191217. Further, as disclosed in JP-A-6-103553, another method is proposed in which the domain wall motion of the soft magnetic underlayer is suppressed by an exchange coupling occurring in connection with an antiferromagnetic layer in which the direction of the magnetic spin is set in one direction.